The Drosophila embryonic epidermis has been an exemplary model system for understanding how transcription factor networks and cell signaling modules are coordinated to pattern tissues and embryos. Recently, it has begun to reveal how those circuits impact the underlying cell biology to drive morphogenesis. Thus, the study of this epithelium holds the promise of a complete understanding of the circuit between developmental cell signaling and the effector proteins that control cellular morphogenesis. This is the overarching focus of this proposal. The remodeling of cell sheets underlies most of morphogenesis during development. For instance, it is the key to extending the axis of the vertebrate body plan, gastrulation, neurulation, and organ formation, in general. When these processes go awry, severe birth defects can result. Surprisingly, it is not understood how specific cell interfaces are selected out for remodeling. Within cell sheets, individual cells often can discern with high fidelity one of their edges from all others, and this is known as tissue polarity. In fact, tissue polarity is likely what assists in the ability of cells to select out a specific edge for remodeling. Tissue polarity is a conserved feature, operating in most if not all epithelia from the invertebrate, Drosophila, through to mammals, and a similarly conserved set of genes is at the center of assigning that polarity. Still, it is unclear how these genes confer polarity, what the effector mechanisms are, and how the polarity circuit is linked to developmental cell signaling. This proposal focuses on two key aspects of tissue cellular morphogenesis: the remodeling of cell sheets;and the assignment of polarity within the plane of the epithelium (so-called "tissue-polarity"). Finally, this work attempts to identify the cell biological mechanisms at the heart of these rearrangements. PUBLIC HEALTH RELEVANCE: Cell polarity is a driving force that shapes embryos and organ systems. Defects in polarity underlie several syndromes, such as ciliary diseases, and deafness. The genes that coordinate polarity were first identified in Drosophila, and their continued study in fruitflies is necessary to understand how they work.